The 23 November 1980 southern Italy earthquake

1983 ◽  
Vol 73 (1) ◽  
pp. 187-200
Author(s):  
Edoardo Del Pezzo ◽  
Giovanni Iannaccone ◽  
Marcello Martini ◽  
Roberto Scarpa
Keyword(s):  
Main Shock ◽  
Southern Italy ◽  
Epicentral Distance ◽  
Tensile Axis ◽  
Multiple Sequence ◽  
Short Period ◽  
Plane Solution ◽  
Hyperbolic Law ◽  
Aftershock Region ◽  
Good Agreement

abstract The seismic activity associated with the catastrophic southern Italy earthquake was monitored by 11 seismic stations operating before this event, within an epicentral distance of 200 km, and by 32 additional short-period seismometers installed soon after the main shock. The hypocenter of this event was located at 40°46′N and 15°18′E, at 16 km depth. The fault-plane solution reveals normal faulting, with tensile axis dipping 18° and oriented orthogonal to the axis of the Apennines chain. This mechanism is in good agreement with the stress pattern inferred from some previous earthquakes and the local seismotectonics. The hypocenter locations of more than 600 aftershocks, with local magnitudes greater than 2.4, show a pronounced alignment extending for about 70 km, oriented north 120° and scattered laterally less than 15 km. These events are mostly concentrated between 8 and 16 km depth. A cluster of aftershocks occurred close to the hypocenter of the main shock covering a region elongated 25 km which corresponds also to the highly damaged area. No significant spreading of the aftershock area with time is observed, but one of the events with higher magnitude (ML = 4.8, 14 February 1981) is displaced 20 km NW from the tip of the aftershock region. The time evolution of the number of aftershocks fits well Omori's hyperbolic law with a decay coeffcient of 1.07 ± 0.06. The possibility of a future delayed multiple sequence of large events, as already observed in the past along the central and southern Apennines, is discussed. In particular, a relatively high seismic potential seems to exist along the northern boundary of the 1980 rupture segment.

The San Salvador Earthquake of October 10, 1986—Seismological Aspects and Other Recent Local Seismicity

1987 ◽  
Vol 3 (3) ◽  
pp. 419-434 ◽  
Author(s):  
Randall A. White ◽  
David H. Harlow ◽  
Salvador Alvarez
Keyword(s):  
Main Shock ◽  
Fault Plane ◽  
Vertical Plane ◽  
Aftershock Sequence ◽  
Central American ◽  
Fault Plane Solution ◽  
San Salvador ◽  
The North ◽  
Volcanic Chain ◽  
Plane Solution

The San Salvador earthquake of October 10, 1986 originated along the Central American volcanic chain within the upper crust of the Caribbean Plate. Results from a local seismograph network show a tectonic style main shock-aftershock sequence, with a magnitude, Mw, 5.6. The hypocenter was located 7.3 km below the south edge of San Salvador. The main shock ruptured along a nearly vertical plane toward the north-northeast. A main shock fault-plane solution shows a nearly vertical fault plane striking N32\sz\E, with left-lateral sense of motion. This earthquake is the second Central American volcanic chain earthquake documented with left-lateral slip on a fault perpendicular to the volcanic chain. During the 2 1/2 years preceeding the earthquake, minor microseismicity was noted near the epicenter, but we show that this has been common along the volcanic chain since at least 1953. San Salvador was previously damaged by a volcanic chain earthquake on May 3, 1965. The locations of six foreshocks preceding the 1965 shock show a distinctly WNW-trending distribution. This observation, together with the distribution of damage and a fault-plane solution, suggest that right-lateral slip occurred along a fault sub-parallel with Central American volcanic chain. We believe this is the first time such motion has been documented along the volcanic chain. This earthquake was also unusual in that it was preceded by a foreshock sequence more energetic than the aftershock sequence. Earlier this century, on June 08, 1917, an Ms 6.4 earthquake occurred 30 to 40 km west of San Salvador Volcano. Only 30 minutes later, an Ms 6.3 earthquake occurred, centered at the volcano, and about 35 minutes later the volcano erupted. In 1919 an Ms 6 earthquake occurred, centered at about the epicenter of the 1986 earthquake. We conclude that the volcanic chain is seismically very active with variable styles of seismicity.

The foreshock sequence of the 1986 Chalfant, California, earthquake

1988 ◽  
Vol 78 (1) ◽  
pp. 172-187
Author(s):  
Kenneth D. Smith ◽  
Keith F. Priestley
Keyword(s):  
Main Shock ◽  
Slip Surface ◽  
Body Waves ◽  
Aftershock Distribution ◽  
Time Period ◽  
Main Shocks ◽  
Short Period ◽  
Local Shear ◽  
Stress Drops ◽  
Foreshock Activity

Abstract The ML 6.4 Chalfant, California, earthquake of 21 July 1986 was preceded by an extensive foreshock sequence. Foreshock activity is characterized by shallow clustering activity, including 7 events greater than ML 3, beginning 18 days before the earthquake, an ML 5.7 foreshock 24 hr before the main shock that ruptured only in the upper 10 km of the crust, and an “off-fault” cluster of activity perpendicular to the slip surface of the ML 5.7 foreshock associated with the hypocenter of the main shock. The Chalfant sequence occurred within the local short-period network, and the spatial and temporal development of the foreshock sequence can be observed in detail. Seismicity of the July 1986 time period is largely confined to two nearly conjugate planes; one striking N30°E and dipping 60° to the northwest associated with the ML 5.7 foreshock and the other striking N25°W and dipping 70° to the southwest associated with the main shock. Focal mechanisms for the foreshock period fall into two classes in agreement with these two planes. Shallow clustering of earthquakes in July and the ML 5.7 principal foreshock occur at the intersection of the two planes at a depth of approximately 7 km. The seismic moments determined from inversion of the teleseismic body waves are 4.2 × 1025 and 2.5 × 1025 dyne-cm for the principal foreshock and the main shock, respectively. Slip areas for these two events can be estimated from the aftershock distribution and result in stress drops of 63 bars for the principal foreshock and 16 bars for the main shock. The main shock occurred within an “off-fault” cluster of earthquakes associated with the principal foreshock. This cluster of activity occurs at a predicted local shear stress high in relation to the slip surface of the 20 July earthquake, and this appears to be the triggering mechanism of the main shock. The shallow rupture depth of the principal foreshock indicates that this event was anomalous with respect to the character of main shocks in the region.

Short-period Lg magnitudes: Instrument, attenuation, and source effects

1983 ◽  
Vol 73 (6A) ◽  
pp. 1835-1850
Author(s):  
Robert B. Herrmann ◽  
Andrzej Kijko
Keyword(s):  
United States ◽  
Epicentral Distance ◽  
P Wave ◽  
Magnitude Scale ◽  
Source Effects ◽  
Anelastic Attenuation ◽  
Basic Conclusion ◽  
Short Period ◽  
Central United States ◽  
Definition Of

Abstract The applicaton of the Nutli (1973) definition of the mbLg magnitude to instruments and wave periods other than the short-period WWSSN seismograph is examined. The basic conclusion is that the Nuttli (1973) definition is applicable to a wider range of seismic instruments if the log10(A/T) term is replaced by log10A. For consistency and precision, the notation mbLg should be applied only to magnitudes based upon 1.0 Hz observations. The mbLg magnitude definition was constrained to be consistent with teleseismic P-wave mb estimates from four Central United States earthquakes. In general, for measurements made at a frequency f, the notation mLg(f) should be used, where m L g ( f ) = 2.94 + 0.833 log ⁡ 10 ( r / 10 ) + 0.4342 γ r + log ⁡ 10 A , and r is the epicentral distance in kilometers, γ is the coefficient of anelastic attenuation, and A is the reduced ground amplitude in microns. Given its stability when estimated from different instruments, the mLg(f) magnitude is an optimum choice for an easily applied, standard magnitude scale for use in regional seismic studies.

The structure of the Cincinnati arch as determined by short period Rayleigh waves

1969 ◽  
Vol 59 (1) ◽  
pp. 399-407
Author(s):  
Robert B. Herrmann
Keyword(s):  
Experimental Data ◽  
Theoretical Model ◽  
Layer Thickness ◽  
Rayleigh Waves ◽  
Layer Model ◽  
Depth Data ◽  
Short Period ◽  
Best Fitting ◽  
Good Agreement

Abstract The propagation of Rayleigh waves with periods of 0.4 to 2.0 seconds across the Cincinnati arch is investigated. The region of investigation includes southern Indiana and Ohio and northern Kentucky. The experimental data for all paths are fitted by a three-layer model of varying layer thickness but of fixed velocity in each layer. The resulting inferred structural picture is in good agreement with the known basement trends of the region. The velocities of the best fitting theoretical model agree well with velocity-depth data from a well in southern Indiana.

The earthquake sequence in Friuli, Italy, 1976

1979 ◽  
Vol 69 (6) ◽  
pp. 1797-1818
Author(s):  
Vittorio Cagnetti ◽  
Vincenzo Pasquale
Keyword(s):  
Main Shock ◽  
Eastern Alps ◽  
Earthquake Sequence ◽  
Laboratory Studies ◽  
Fault Movement ◽  
Stress Changes ◽  
Entire Sequence ◽  
Strain Release ◽  
Largest Aftershock ◽  
Aftershock Region

abstract The seismic activity of the May 6, 1976 Friuli earthquake has been investigated. It provides clear evidence of internal clustering of shocks, with the largest aftershocks being followed by their own series of aftershocks. Late large aftershocks with their own aftershock series occurred 4 months after the main shock, when aftershocks had subsided. Thus, in the entire series of aftershocks, six phases of strain release are found, and part of the aftershock region is not included in the aftershock volume of the main shock. All this indicates that a few aftershocks are at least partially independent from the main shock. The value of b is estimated for the entire sequence and for the separate phases; during the activity, b shows an increase after the main shock, a decline immediately before the largest aftershock, and a second increase immediately afterward. This can be explained in terms of stress changes, and is consistent with laboratory studies of rock deformation. The compressive stress is perpendicular to the Eastern Alps, and may be considered as the principal cause of the earthquake sequence. The solution of the main shock of the sequence is a reversed fault movement, unlike most of the mechanisms in the focus of the earlier Friuli earthquakes which are of the transcurrent type.

Local Richter magnitude and total signal duration in southern California

1973 ◽  
Vol 63 (5) ◽  
pp. 1809-1827 ◽  
Author(s):  
Charles R. Real ◽  
Ta-Liang Teng
Keyword(s):  
Southern California ◽  
Epicentral Distance ◽  
Total Duration ◽  
Signal Duration ◽  
Local Magnitude ◽  
Total Signal ◽  
Source Effects ◽  
Short Period ◽  
Seismic Networks ◽  
Southern California Region

abstract Seismograms of 320 earthquakes (1,486 observations) from short-period seismometers occurring from January 1969 to April 1971 and 91 earthquakes (257 observations) during 1971 have been used to establish a relationship between total signal duration and the local Richter magnitude for the CIT and BHSN telemetered seismic networks in southern California. The data have been fitted using regression analysis to relationships of the form M τ = C 0 + C 1 log ⁡ τ + C 2 Δ M τ ≦ 3.8 M τ = C 0 + C 1 ( log ⁡ τ ) 2 + C 2 Δ M τ > 3.8 where τ is the total duration in seconds and Δ is the epicentral distance in kilometers. These relations explain up to 88 per cent (CIT) and 94 per cent (BHSN) of the variation in the data and yield magnitudes having standard deviations as low as 0.15 (CIT) and 0.14 (BHSN) magnitude units. It has been found that the local magnitude based on signal duration is relatively insensitive to variations in azimuth and source effects. In view of the limited distribution and low magnifiation of the Wood-Anderson torsion seismometer, and the previously recognized problems of “saturation” and instrument response associated with the amplitude technique, it is concluded that the method of duration applied to vertical short-period seismograph records will greatly improve the assignment of local magnitude to earthquakes in the southern California region.

Aftershocks and surface faulting associated with the intraplate Guinea, West Africa, earthquake of 22 December 1983

1987 ◽  
Vol 77 (5) ◽  
pp. 1579-1601
Author(s):  
C. J. Langer ◽  
M. G. Bonilla ◽  
G. A. Bollinger
Keyword(s):  
West Africa ◽  
Focal Mechanism ◽  
Main Shock ◽  
Strike Slip ◽  
Fault System ◽  
Epicentral Area ◽  
Lateral Strike Slip ◽  
Focal Mechanism Solutions ◽  
Short Period ◽  
East West

Abstract This study reports on the results of geological and seismological field studies conducted following the rare occurrence of a moderate-sized West African earthquake (mb = 6.4) with associated ground breakage. The epicentral area of the northwestern Guinea earthquake of 22 December 1983 is a coastal margin, intraplate locale with a very low level of historical seismicity. The principal results include the observation that seismic faulting occurred on a preexisting fault system and that there is good agreement among the surface faulting, the spatial distribution of the aftershock hypocenters, and the composite focal mechanism solutions. We are not able, however, to shed any light on the reason(s) for the unexpected occurrence of this intraplate earthquake. Thus, the significance of this study is its contribution to the observational datum for such earthquakes and for the seismicity of West Africa. The main shock was associated with at least 9 km of surface fault-rupture. Trending east-southeast to east-west, measured fault displacements up to ∼13 cm were predominantly right-lateral strike slip and were accompanied by an additional component (5 to 7 cm) of vertical movement, southwest side down. The surface faulting occurred on a preexisting fault whose field characteristics suggest a low slip rate with very infrequent earthquakes. There were extensive rockfalls and minor liquefaction effects at distances less than 10 km from the surface faulting and main shock epicenter. Main shock focal mechanism solutions derived from teleseismic data by other workers show a strong component of normal faulting motion that was not observed in the ground ruptures. A 15-day period of aftershock monitoring, commencing 22 days after the main shock, was conducted. Eleven portable, analog short-period vertical seismographs were deployed in a network with an aperture of 25 km and an average station spacing of 7 km. Ninety-five aftershocks were located from the more than 200 recorded events with duration magnitudes of about 1.5 or greater. Analysis of a selected subset (91) of those events define a tabular aftershock volume (26 km long by 14 km wide by 4 km thick) trending east-southeast and dipping steeply (∼60°) to the south-southwest. Composite focal mechanisms for groups of events, distributed throughout the aftershock volume, exhibit right-lateral, strike-slip motion on subvertical planes that strike almost due east. Although the general agreement between the field geologic and seismologic results is good, our preferred interpretation is for three en-echelon faults striking almost due east-west.

Radial Velocity Variations in Cool Supergiants

1984 ◽  
Vol 88 ◽  
pp. 283-288
Author(s):  
Hugh C. Harris
Keyword(s):  
Radial Velocity ◽  
Main Sequence ◽  
Short Period ◽  
Cool Stars ◽  
Synchronous Rotation ◽  
Red Supergiants ◽  
Velocity Variations ◽  
Orbital Periods ◽  
Low Amplitude ◽  
Good Agreement

AbstractA survey of F, G, and W supergiants has been carried out with the DAO radial velocity spectrometer, an efficient instrument for detecting low-amplitude velocity variations in cool stars. Observations of 78 stars over five seasons show generally good agreement with OORAVEL results for spectroscopie binaries. The majority of supergiants show low-amplitude variability, with amplitudes typically 1 to 2 km s−1. The width of the cross-correlation profile has been measured for 58 supergiants. It reveals 14 stars with unusually broad lines, indicative of rotation velocities of 15 to 35 km s−1. Several have short-period binary companions and may be in synchronous rotation. The other broad-lined stars are apparently single or with long orbital periods; they may be making their first transition from the main sequence to become red supergiants.

scholarly journals V471 Tauri and SuWt 2: The Exotic Descendants of Triple Systems?

2002 ◽  
Vol 187 ◽  
pp. 239-243 ◽  
Author(s):  
Howard E. Bond ◽  
M. Sean O’Brien ◽  
Edward M. Sion ◽  
Dermott J. Mullan ◽  
Katrina Exter ◽  
...  
Keyword(s):  
Main Sequence ◽  
Central Star ◽  
High Mass ◽  
Eclipsing Binary ◽  
Triple Systems ◽  
Common Envelope ◽  
Short Period ◽  
Efficiency Parameter ◽  
Hydrodynamical Simulations ◽  
Good Agreement

AbstractV471 Tauri is a short-period eclipsing binary, and a member of the Hyades. It is composed of a hot DA white dwarf (WD) and a cool main-sequence dK2 companion. HST radial velocities of the WD, in combination with the ground-based spectroscopic orbit of the K star, yield dynamical masses of MWD = 0.84 and MdK = 0.93 M⊙. During the UV observations we serendipitously detected coronal mass ejections from the K star, passing in front of the WD and appearing as sudden, transient metallic absorption. Eclipse timings show that the active dK star is 18% larger than a main-sequence star of the same mass, an apparent consequence of its extensive starspot coverage. The high Teff and high mass of the WD are paradoxical: the WD is the most massive in the Hyades, but also the youngest. A plausible scenario is that the progenitor system was a triple, with a close inner pair that merged after several × 108 yr to produce a single blue straggler. When this star evolved to the AGB phase, it underwent a common-envelope interaction with a distant dK companion, which spiraled down to its present separation and ejected the envelope. The common-envelope efficiency parameter, αCE, was of order 0.3–1.0, in good agreement with recent hydrodynamical simulations.SuWt 2 is a southern-hemisphere planetary nebula (PN) with an unusual ring-shaped morphology. The central star is an eclipsing binary with a period of 4.9 days. Surprisingly, the binary is composed of two main-sequence A-type stars with similar masses of ~ 2.5 M⊙. We discuss scenarios involving a third companion which ejected and ionizes the PN.WeBo 1 is a northern PN with a ring morphology remarkably similar to that of SuWt 2. Although we hoped that its central star would shed light on the nature of SuWt 2, it has proven instead to be a late-type barium star!

scholarly journals Fractal analysis for the ULF data during the 1993 Guam earthquake to study prefracture criticality

2006 ◽  
Vol 13 (4) ◽  
pp. 409-412 ◽  
Author(s):  
Y. Ida ◽  
M. Hayakawa
Keyword(s):  
Fractal Analysis ◽  
Main Shock ◽  
Epicentral Distance ◽  
Sense Of Self ◽  
Low Frequency ◽  
Large Earthquake ◽  
Self Organized ◽  
Self Organized Criticality ◽  
Electromagnetic Signature

Abstract. An extremely large earthquake (with magnitude of 8.2) happened on 8 August 1993 near the Guam island, and ultra-low-frequency (ULF) (frequency less than 1 Hz) electromagnetic fields were measured by 3-axis induction magnetometers at an observing station (with the epicentral distance of 65 km) with sampling frequency of 1 Hz. In order to study electromagnetic signature of prefracture criticality, we have undertaken the fractal (mono-fractal) analysis by means of the Higuchi's method for the ULF data during the 1993 Guam earthquake. Then, it is found that the fractal dimension exhibits five maxima 99, 75, 52, 21, and 9–4 days before the earthquake main shock, which suggests the ULF electromagnetic signature of nonlinear evolution (in the sense of self-organized criticality) taking place in the lithosphere just before the 1993 large Guam earthquake. That is, there take place step-like changes in the lithosphere during the long-term of the order of several months before the main shock.

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